Method for carburizing in a continuous furnace



Oct. 4, 1960 o. E. CULLEN ETAL 2,955,062

METHOD FOR CARBURIZING IN A CONTINUOUS FURNACE original Filed Feb. 27. 1952 4 Sheets-Sheet 1 Oct. 4, 1960 o. E. CULLEN ETAL 2,955,062

METHOD FOR CARBURIZING IN A CONTINUOUS FURNACE 4 Sheets-Sheet 2 Original Filed Feb. 27, 1952 udn-30u02 UNIF INVENTORS QCUE/V BY ICO/VL'Y Oct. 4, 1960 o. E. cULLEN ETAL 2,955,062

METHOD FOR CARBURIZING 1N A CONTINUOUS FURNACE Original Filed Feb. 27, 1952 4 Sheets-Sheet 3 V w "1 U o ml oo 2 d 1 2 d 2 o S 3 a: z 3 x unot 'i 3 3 i' a 2 2 3 0 E o G a. U n [D o, i Q

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METHOD FOR CARBURIZING IN A CONTINUOUS FURNCE original Filed Feb. 2v. 1952 4 sheets-sheet 4 United States Patent F METHOD FOR CARBURIZING IN A CONTINUOUS FURNACE Orville E. Cullen, Toledo, Ohio, and John E. Conley, Berkley, Mich., assignors, by mesne assignments, to Midland Ross Corporation, Cleveland, Ohio, a corporation of Ohio Original application Feb. 27, 1952, Ser. No. 273,696. Divided and this application Mar. 29, 1956, Ser. No. 580,425

4 Claims. (Cl. 14S-16.5)

This invention relates to `a continuous carburizing furnace and method for continuous carburization.

In the art of carburizing it is well known to use solid material such as charcoal for pack carburizing, and it is known to vary the solids used to obtain variations in the character of the resultant carburized case. It is customary to blend fresh solids with used, or spent, solids for that purpose.

With the advent of gas carburizing processes the mechanics of carburization have become more understandable, and noteworthy advances have been made both in the art of carburization and of carbon restoration to partially decarburized work. These developments stemmed largely from the use of atmospheres consisting essentially of N2, CO, CO2, H2, H2O and CH4 or other hydrocarbon equivalents of CH4. Using such atmospheres soot deposition in a furnace can be largely eliminated, as taught by Turin Patent 2,287,651, and the carbon potential (hereinafter defined) can be controlled.

By proper control of carbon potential and furnace temperature, the surface carbon content of carburized work may be controlled, with resultant desirable metallurgical results, but of course the rate of absorption of carbon into the steel is correspondingly reduced when the carbon potential is reduced, it being understood that the driving force for absorption of carbon into the work is proportional to the excess of the atmosphere carbon potential over the surface carbon content of the work.

Carbon potential of a gas, as used herein, and in the appended claims, indicates the carbon content to which that gas will carburize steel if equilibrium is reached; it is customarily measured in percent of carbon in thin strips of steel which have been brought to substantial equilibrium with the gas atmosphere and have a sub-v stantially uniform carbon content throughout the strip. Thus, a gas having a carbon potential of 0.80 percent at T F. would be in equilibrium with steel containing 0.80 percent of carbon at T F., would carburize steel containing 0.70 percent of carbon at T F., and would decarburize steel containing 0.90 percent of carbon at T F. Carbon potential is a function of temperature, however, so that a gas having a carbon potential of 0.80 percent at T F. would have a carbon potentialV other than 0.80 at either a lower or a higher temperature. In general, at least within the austenitic range of the work, the carbon potential of a gas of given composition increases with decreases in temperature, Furthermore, known gases having a carbon potential within the range usually required for conventional carburization, i.e., 0.60 to 1.40 percent, at practical carburizing temperatures, decompose badly and deposit large quantities of soot at lower temperatures within the range to which work must often be cooled prior to discharge from the furnace.

The present invention provides in continuous apparatus for carburizing ferrous metal work, an optimum distribution of carbon in the work, and discharge of thev Work at ra desired temperature and after a minimum of time in the furnace for the depth of carburized case desired. The invention can be fully understood from the description which follows, and by reference to the accompanying drawings, in which,

Fig.` 1 is an elevation, partially in section, showing mechanical features of a continuous furnace for practicing the invention;

Fig. 2 is a sectional view along the line 2 2 of Fig. l; Fig. 3` is a sectional view along the line 3 3 of Fig. 1;

Fig. `4 is a partially diagrammatic plan view of a.V furnace of Fig. 1 showing means for controlling the atmosphere therein as hereinafter described;

Fig. 5 is a partially schematic view showing a way of eecting temperature control ina furnace of the invention; y

Fig. 6 is a graph showing carbon distribution achieved according to the invention and according to continuousV prior art carburization methods;

Fig. 7 is a Vschematic flow diagram showing the steps in the process of the invention; and

Fig. 8 is a schematic representation showing generally the temperature relationships in the process of the inven tion.

surface carbon content and case depth, effecting an improved carbon distribution in the work, and discharging the carburized vwork at the desired discharge temperature. The apparatus comprises a controlled atmosphere furnace having a carburizing zone and a diffusing zone, which furnace has means for conveying the work into and through the carburizing zone, and through and from the diffusing zone, means for controlling the furnacey temperature to provide, within the carburizing zone, a carburizing region having a substantially constant ternperature Vwithin the-austenitic range of the work, and a temperature Within the austenitic range of the work in at least a portion of the diffusing zone, means for creat-V ing and maintaining an atmosphere having a high carbonV potential in the carburizing zone, and means for creating and maintaining in the diffusing zone an atmosphere having a substantially lower carbon potential, at least as high as the carbon content of the interior of the workl butI not substantially higher than the desired surface carbon content. Preferably, the carbon potential of the atmosphere in the carburizing zone, at the temperature of the carburizing region, is from 1.00 percent to 1.40

percent, and most desirably it is approximately equal toV the carbon content of austenite saturated with carbon'at the temperature of the carburizing region. It is valso preferred that the carbon potential of the atmosphere inV the'dilfusing zone relative to the work be. from (X 0.30) percent to X percent where X is the desired surface carbon content of the work; most desirably, it is approXimately X percent.

The furnace shown in Figs. 1, 2, 3, and 4 of the draw-- ings `is Yacontinuousapparatus comprising a controlled atmosphere furnace indicated generally at 11, means indicated generally at 60 in Fig. 4 for creating and maintaining in a carburizingzone 12 of the furnace an at-V mosphere of any desired carbon potential, and meansy able by doors 25 and 26. The doors 21 and 20 are raisedor lowered by chains 29 and 30 driven by sprockets 31 and 32 rotated in any conventional way, not illustrated.

i f Patented Oct. 4, 1360` According to the invention, continuous apparatusv is'A utilized for carburizing ferrous met-al work to a desired- 3 The chains 29 and 30 are made incapable of opening either of the doors 2i) and 25 or 21 and 26 until the other is closed, and similarly the inner doors and 21 may n ot both be opened Vat the same time. This is preferably accomplished by electrical control interloclgs, as is welll known. Conduits 35 and 36 with valves 33 and 34 provide passages through which-atmospheregis discharged from either end of the furnace into the corresponding vestibule. Second conduits 37 and 38 provide atmosphere exhausts from each vestibule. 'lfhe conduits 3.5 and 37 provide atmosphere exhausts fromV the charge end of the furnace, and the conduits 36 and 38-provide atmosphere exhausts from the discharge end of the furnace.

y The furnace 11 is enclosed by ashell 39.` I n the embodiment of the apparatus of the invention shown in Fig., 1 the furnace isheated by upper radiant Vtubes 40, and lower radiant tubes 41 in which combustible gases are burned. Temperature in the furnace may be controlled by a series of recording type Ytemperature controllers 45 see Figs. 4 and 5)y each of which is operablyattached to oneA of a series of thermocouples 46'gappropriately positioned in the furnace in tubes 47, preferably inserted through the roof .thereof with one thermocouple to each of several zones in the furnace., The controllers operate solenoid actuated valves 48 which they open wider'when the voltage from a thermocouple 46 indicates a furnace temperature lower than that prescribed, and close partial: `ly, when the voltage indicates the reverse. The valve 4S regulates the rate of ow of an air-natural gas mixture to the radiant tubes 40 and 41; the combustion of this mixture heats the tubes, and, ,through them, .the furnace. Alternatively, the furnace 11 could be heated by electric resistance elements, or by direct fired burners, asV in a muftle furnace, not illustrated, in a conventional manner.

The furnace is provided, also, with a suitable conveyor (Fig. 1) consisting of a series of operably aligned driven rolls 50. The rolls 50 are conveniently driven` (see Fig. 2) by chains 51 which mesh with sprockets 52 and 53 attached, respectively, to the rolls and to shafts S4 of motors 55. It is usually feasible and preferred that a single chain 51 mesh with and drive several sprockets 52.

In the furnace shown in Figs. 1, 2, 3 and, 4 the furnace 11 is divided into sections in any convenient manner as by a series of jack arches 57 and solid piers thereunder which together serve to substantially divide said sections. In each of the sections is positioned a circulating fan 58 which moves furnace atmosphere in a vertical direction within the respective section. The use of such circulating fans is advantageous, although not essential, as it mini'- mizcs the possibility of a stagnant atmosphere pocket of dilferentcomposition from that contemplated.

The means 60 for creating and maintaining in the carburizing zone 12 an atmosphere of a desired carbon potential consists of a conduit 61 supplied with a carrier gas,

hereinafter identified, from a generator or from storage,

not illustrated. The gas passes through the conduit 61 in the direction of the arrow (Fig. 4), through a regulating valve 62, which can be operated either manually or automatically, thence into a iiow meter 63 where the rate of gas ow is determined visually or automatically, and into a mixer 64. A CH., gas, which Vterni is used herein to :include natural gas and relatively pure methane, ethane, propane and other hydrocarbons and oxyhydroc Icarbons that are methane equivalents in that they are known enriching gases for carburizing, can be mixed with the carrier gas in the mixer in any desired proportion, usually not more than 5 percent of the total gas, calculated as4 methane. Such mixing, Aif desired, is accomplished by passing the CH4 gas from storage or from aY mainnot illustrated, through Va conduit 65, in the direction of the arrow, a control valve 66, a ow Ameter 67 and thence into the. mixer '64. The treating gas of desired 'composition and Carbon potential is blended in the mixer ,64 and ows from there into a manifold 68, and throughconduits. 69 into -the treating zone. In the. etnbodiment of the invention shown in Fig. 4, the means 60 is duplicated on either side of the furnace 11. The rate at which atmosphere is introduced into the carburizing Zone should be such that a small superatmospheric pressure is maintained withinthe carburizing zone, and should be sufciently high that 'substantial reduction in the carbon potential'of theV atmosphere'does not occur because of reaction and absorption of carbon by Vworkin the furnace.

The means 70 for introducing an atmosphere into the diffusing zone receives a carrier gas from the regulating valve o2, which gas flows through a line 75, in the direction of the arrow, through a control valve 76, and through a ow meter 77, and into a mixer 78. The means 70 also receives a potential controlling gas (usually an oxidizing gas as hereinafter discussed) which flows from storage (not illustrated) through a line inthe direction of the arrow, through a control valve 81, through a ow meter 82, and into the mixer 78. The carrierpotential controlling gas mixture iiows into the diffusing zone from the mixer 78 through a manifold 85 and conduits 86. In the furnace illustrated the means 70 is duplicated on both sides of the'furnace, and Vsupplies atmosphere to two sections of the furnace. Since diffusion occurs at a relatively rapid rate it is practical to supply the'potential controlling gas only to the last section, for example to the 1/6 or 1A; of the furnacey nearest the discharge end, unless work is to be cooled to a low temperature before itis discharged.

It has been found that the carbon potential` of a ea-rburizing `gas is reduced by Imixture therewith of an oxidizing gas such as air H2O, or CO2, or by dilution thereof with a substantially neutral gas. Accordingly, the apparatus described in the preceding paragraphs is operable -to maintain the carbon potential of the atmosphere -in the diffusing zone -atfany desired level below that in the carburizing zone. The 4amparatus can be so operated that substantially all atmosphere introduced through the means 60 into the treating zone flows toward the charge en-d of the furnace and is exhausted through the conduits 35 and 37. Substantially all atmosphere introduced into the diffusing zone then iiows toward the discharge end of the furnace, and is exhausted through the conduits 36 and 3S. The division of flow of atmosphere from the furnace chamber through the respective ends of the furnace is preferably accomplished'by proper relative sizing of the exhaust con-duits 37 and 38, conduit 39 being smaller when less atmosphere is to flow from the discharge end of the furnace. Positioning one of the dividing `arches 57 between the carburizing zone 12 and the diffusing Zone 14 assists in isolating the atmosphere in the two sections of the furnace and :also assistsin temperature control when, as is generally preferred when ,the work is to be directly quenched from the furnace, the temperature in the diffusing zone is lower than the temperature in the carburizing zone.

Under rthese operating conditions :the carbon potential of the atmo-suhere introduced into the carburizing zone is adjusted to the desired value, usually between 1.0() percent .and 1.4() percent, by suitable regulation of the constituents thereof. Similarly, the carbon potential of the atmosphere introduced into the diffusing zone is adjusted to the desired value, not lower than the internal carbon content of `the work, and not higher than the desired Vsurface carbon content.

The apparatus described is not limited to operation in the manner described in the preceding paragraph. It is feasible 4to introduce into the carburzing zone a volume of gas appreciably greater than the volume exhausted from Ithe furnace through the conduits 35 and 37. A corresponding decrease is then made in the volume of gas introduced into the diffusing zone, and the proportion of potential controlling gas therein is increased. As a result, there vis an appreciable flow of atmosphere from the carburizing zone into and through the diffusing zone;l this atmosphere is diluted in the diffusing zonewith gas of carbon potential lower than desired therein. By blending of the two gases, in the diffusing zone, an atmosphere of desired carbon potential is created and maintained therein.

It is clear from the above that the apparatus can be operated to avoid appreciable mingling of the atmospheres in the carburizing and diffusing zones, or so thatY atmosphere 4from the former may flow into the latter where the desired carbon potential is created and maintained by mixture therewith of a diluting gas, preferably of a gas diluted with an oxidizing gas. iowever, it is preferable to -avoid ow of atmosphere from the diffusing zone to the carburizing zone. Although it would be possible to operate in such way, raising the carbon potential in the carburizing zone from that required in the diffusing zone to a value within the range from 1.00 percent to 1.40 percent, as is usually preferred therein, 1s appreciably more dificult than effecting a corresponding decrease, as described, in the diffusing zone.

In practicing Ithe method of the invention, it is feasible to heat ferrous metal -work at a 4rapid rate. If suicient provision is made for heating the charge end of the c-arburizing zone 'a substantially constant temperature can be maintained throughout this entire zone. Such temperature should be within the austenitic range of the work, because carburization is practical only at temperatures where the work being carburized is austenitic. A portion of-the carburizing zone, called a carburizing region, herein, must therefore, be maintained at a temperature within the -austenitic range of the Work; to avoid variations in carbon potential within the carburizing region, the temperature -thereof should also be approximately constant. If desired, the temperature of the charge end of the carburizing zone can be lower than the temperature of the carburizing region; the charge end then constitutes a heating region of the carburizing zone.

In general, the temperature within the diffusing zone 14 should be within the austenitic range of the work, such as l525 F. for most carb-urizing grade steels, at least in the portion of the diffusing zone adjacent the carburizing zone. It is usually preferred that no temperature in the diffusing zone be higher, and most preferred that all temperatures therein be lower, than the carburizing region temperature. The diffusingl zone tempera-ture at the discharge end 4off Ithe furnace is usually. not higher than 1550 F., and can be as low as room temperature although preferably, it is from 1400 F. to 1550 F. from which temperature it is usually quenched. A temperature within the austenitic range of the work is required in a portion of the diffusing zone in order that carbon can diffuse from the `surface of the work into the interior thereof as hereinafter described.

The improved result achieved by carburizing ferrous metal work according to the process of the invention is shown graphically in Fig. 6 of the drawings. Fig. 6 is a diagram on which is plotted carbon content of the work, expressed as percent by weight, against distance from the surface of the work. Curve I represents the an atmosphere of substantially constant carbon potential, of high final potential. The curve is generally hyperbolic, approaching t-angency with the abscissa at 1.20 percent carbon, and approaching tangency with a line representing 0.20 percent carbon, the carbon content condition of Work carburized in the usual manner, by

a carbon content of 1.20 percent, while Curve' shows acarbon content of 0.76 percent.

The .difference between work having a carbon distribu-` tion represented by Curve I (called Work I, for convenience) and work having-a carbon distribution represented by Curve II (called Work Il, for convenience) is highly significant to manufacturers who use ferrousv metals. vFor example, .many parts require final finishing .after carburization; Work I is much more difficult .to finish than is Work II because the metal that must be removed is of extremely high carbon content. Furthermore, it has been found that Work I frequently con-l tains free carbides, which 4are considered to be deleterious. Also, in steels containing small amounts of boron, high carbon content is disadvantageous, while steel of a carbon content from .about 0.70 percent to about 0.90 per' cent reacts, upon quenching, in the manner of highly. alloyed steels (see. S.A.E. Journal, July 1951, pages 46-' 52). Heretofore, so far as is known, no continuous furnace has been produced capable of carburizing ferrous metal work to provide a carbon distribution as shown by Curve II. When work is carburized in apparatus of the invention, or according .to the method of lthe invention, a carbon distribution as shown by Curve II lmay be achieved. 1

4 In a preferred embodiment the method of the invention comprises passing ferrous metal work successive1y,. first through 1a carburizing zone, where the surface of the work is substantially saturated with carbon at a temperature from :about 1350 F. to l800 F. by treatment with an atmosphere having a carbon potential from 1.00 percent to 1.40 percent at the saturating temperature, and then passing the work into a diffusing` zone where the surface carbon of .the work is reduced` by diffusion at least in part .at a temperature within the austenitic range of the work in an `atmosphere havingv a lower carbon potential not substantially greater than art) have approximately the lfollowing volume composi-A tion: 12 to 25 percent CO, l5 to 50 percent H2, traces, of CH4, H2O, and CO2, and a balance of at least 20 percent of N2. 'Mos-t desirably, the carrier gas contains about 21 volume percent of CO, 38 volume percent of H2, .traces of methane, CO2 and H2O, and the balance N2. It is sometimes desirable to enrich the carrier gas by additions of up to about 5 volume percent of a CH., gas. The percentage of CH, .gas is calculated on the basis' of methane, or carbon. It is `also desired that gas' iiows be so regulated that a small positive pressure is maintained in the zones in which carburizing is accomplished according to the invention. It is preferable that carbon potential of the carrier gas be controlled by control of the dew-point thereof, other components being held substantially constant.

The extent of dilution with a neutral gas required to give a desired carbon potential in the diffusing zone can be determined as described in ran application entitled' Continuous Heat Treat Furnace. The amount of an oxidizing gas required can be determined experimentally or can be calculated Ifrom known thermodynamic relations. It is practical to accomplish carburizing according toi the invention when residence times of from 2 to 14: hours in Ithe car-burizing region of the carburizing zone, .and of from 1/2 to 6 hours in theV diffusing zone'arev provided.

This is a division of application Serial No. 273,696, filed February 27, 1952, now abandoned.

The following example is presented solely to clarify the disclosure of the invention and is to be construed as illustrative only, and not as limiting the invention:

Example l Cold rolled steel bar stock-*having interior carbon contentof .about 0;20l perent'wascarburized in a furnace .substantially "a's illustrated 'in Pigs. 1, 2, 3 'and 4. The tempera'tureA controls were set to provide a temperatureofnabout 1650* Ffin the irst 1A of thecarbun'zing zone, 1700 Flin therest 'of the'c'arburiring zone, which constitutedthe carburizing region, land-.temperature decre'as'esiin V.the direction of vwork travel through the diffusing zoneirom `about 1700 F. toaabont 1500 F. An enriched carrier, 'carburizing gas atmosphere was then introducedinto the carburiaing zone through the means 60 fand the `conduits '69 'at arate of about `Z100 cubic feet per -hourythe approximate volume-composition of the carrier gas Vvwas 'asV follows: 'Hm 38 percent, CO, 21.0 percent, CO2, trace, CH4, 0.2 percent, dew-point F.,'bala'nce N2. `Natural gas, relatively pure methane, was usedras'anenriching gas at a rate of 30 cubic feet per hour.` The carbon potential of this gas is about 1:40 percent at 1700 F. The ow of the above gas was maintained at about the indicated rate throughout carburization. A potential controlling .gas wasprepared by blending 400 cubic feet per hour of `the above carrier gas with 115 cubic feet per hour 'of air, and was admitted to the diffusing zone at a rate of about 415 cubic feet. per hour. The .furnace pressure was slightly superatmosphericta :fraction of an inch 'of water) throughout the operation. `Steel bar stock discharged Vby the furnacefafter 'residence times of about 8 hours in the carburizing zone, including ''hours in Vthe carburizing region, and 2 hours in the diiusing zone was `found to bersubstant-ia-lly free of deposited carbon, `and to have arcarbo'n distribution substantially 'as shown by Curve II of Fig. 6.

' nAs isindicatedabove carbon potential can -be calculated :as/function of temperature, for a'gas of any given composition, from .thermodynamic relationships. However, -inusing .apparatus of the invention, or in' practicing themethod of the invention, it is seldom possible .to be cert-ainof precise atmosphere composition at all points, particularly in the dilusing'zone, even by analizing samples. Accordingly, it is usually preferable that VcarbonV potential, 'particularly in the diffusing zone, be determined experimentally: namely, by determining the carbon content Ito `which thin strips 'are carburized at substantial equilibrium, under la given -set of conditions. If the carbon potentialis higher or lower .than desired in the car-buri'zing zone,kthe methane in the enriched carrier gas canbe lowered or raised, respectively, Ato make suitable compensation. f Iii the carbonpotential in the diifusing zone `is higher or'lower than desired, Ithe amount of potentialrcontrolling Agas can be raised or lowered, respectively, to make suitable compensation.

HIt is usually preferable that the carburizing region :temperature befroin 1550 F. to l800 F.,rnost desirably froml650 P. to 1725 F.

Having described the invention, we claim:

l. 'A continuous method for'carburizing ferrous metal workto'a'surface carbon content of X percent wherein Xis `frorn 0.60 percent to 1.00 percent which comprises passing the work, successively, iirst through a carburizing zone who-se temperature is controlled to provide a substantially `constant temperature within the austenitic range of the work in a carburizing region within the carburizing zonefandthen through a diffusing zone whose temperature is :controlled to provide a temperature within the austeniticV range' of the work in at least a portion of the diffusing zone,.passing'through the carburizing zone, substantially countercurrent to the work, an atmosphere having a carbon potential, at the temperature of the carburizing region, of from 1.00 percent to 1.40 percent, passing through the diusing zone, in the direction of work travel, an atmo'sph'ere having a carbon potential,at the' temperature 'of the diiufsing zone, not greater than X percentand' not les's than the carbon content of thefintr'ior of the Work, and regulating the rate of work travel 'to provide residence times ci from 2 to 14 hours in the carburizing'region of the carburizing zone, and from 1/2 to 6 hours' inthe diifu'sing zone. p

2. A continuous r'nethod as claimed inlclaim lin which the temperatures within thecarburizing zone'are within the range from l550 F. to 1800 F., and the temperatures within the diiusing zone are within the 'range from l700 F. to 12.00 F.

3. A continuous 'method of icarburizing ferrous metal Work'to 'a surface carbon content of X percent which comprises passing the work, successively, rstthrough a carburizing zone whose temperature is controlledl to provide a substantially constant temperature within the austenitic range of the work in a carburiz'ing region Within the carburizing zone, and then through a diffusing zonevvhose temperature is controlled to provide a temperature within the austenitic range of the work in atleast aportion-of the diffusing zone, and no temperature higher than that yin the carbu'rizing region, passing through the lcarbinizing zone, substantially Vcountercurrent to the work, an atmosphere blended from a carburizing gas and not more than 5 percent of a CH4 gas, which atmosphere has a carbon potentiaL'at the temperature of the carburizing region, oi from 1.00 percent to 1.40 percent, passing through'the diffusing zone, in the direction of work travel, an atniosphere consisting of a blend of the Ycarburiz'ing gas with an oxidizing gas of the group consisting of H2O, air, and CO2, in such proportions that the-carbon potential of the atmosphere, :at the temperature of the diffusing zone, is not greater than X percent, and not less than the carbon content ofthe interior of the Work, and regulating the rate of work travel to provide residence times of from 2 to 14 hours in the carburizing region of the carburizing zone, and from 1/2 to 6 hours in the diusing zone.

4. A continuous method for carburizingferrous metal Work to a surface carbon content of X percent wherein X is from 0.60 percent to 1.00 percent which comprisesV passing the Work, successively, first through a carburizing zone whose temperature is controlled to provide 'a substantially constant temperature within the austenitic range of the work in a carburizing region within the 'carbur-izing zone, and then through -a diffusing zone whose temperature is controlled to provide a temperature within the austenitic range of the work in at least a portion'of the diiusing zone, passing throughthe carburizing Zone, substantially countercurrent to the work, an atmosphere having a carbon potential, at the temperature of the carbu'rizing region, of from 1.00 percent to 4the percent for saturatedV austente in the Work, passing through the diffusing Zone, in the direction of Work travel, an atmosphere having acarbon potential, at the temperature of the diffusing zone, not greater than X percent and not'less than the carbon content of the interior of the work, and regulating the' rate of Work travel to provide residence times of from 2 to 14 hours in the carburizing region of the carburizin'g zone, and from 1/2 to 6 hours, in the diifusing zone.

References Cited in theiile of this ,patent UNITED STATES PATENTS Turin June 23,1 l942 OTHER REFERENCES 

1. A CONTINOUS METHOD FOR CARBURIZING FERROUS METAL WORK TO A SURFACE CARBON CONTENT OF X PERCENT WHEREIN X IS FROM 0.60 PERCENT TO 1.00 PERCENT WHICH COMPRISES PASSING THE WORK, SUCCESSIVELY, FIRST THROUGH A CARBURIZING ZONE WHOSE TEMPERATURE IS CONTROLLED TO PROVIDE A SUBSTANTIALLY CONSTANT TEMPERATURE WITHIN THE AUSTENITIC RANGE OF THE WORK IN A CARBURIZING REGION WITHIN THE CARBURIZING ZONE, AND THEN THROUGH A DIFFUSION ZONE WHOSE TEMPERATURE IS CONTROLLED TO PROVIDE A TEMPERATURE WITHIN THE AUSTENITIC RANGE OF THE WORK IN AT LEAST A PORTION OF THE DIFFUSING ZONE, PASSING THROUGH THE CARBURIZING ZONE, SUBSTANTIALLY COUNTERCURRENT TO THE WORK, AN ATMOSPHERE HAVING A CARBON POTENTIAL, AT THE TEMPERATURE OF THE CARBURIZING REGION OF FROM 1.00 PERCENT TO 1.40 PERCENT, PASSING THROUGH THE DIFFUSING ZONE, IN THE DIRECTION OF WORK TRAVEL, AN ATMOSPHERE HAVING A CARBON POTENTIAL, AT THE TEMPERATURE OF THE DIFFUSING ZONE, NOT GREATER THAN X PERCENT AND NOT LESS THAN THE CARBON CONTENT OF THE INTERIOR OF THE WORK, AND REGULATING THE RATE OF WORK TRAVEL TO PROVIDE RESIDENCE TIMES OF FROM 2 TO 14 HOURS IN THE CARBURIZING REGION OF THE CARBURIZING ZONE, AND FROM 1/2 TO 6 HOURS IN THE DIFFUSING ZONE. 